PURPOSE: To stabilize the concentration of sulfur oxides and observe an environmental criteria by a method wherein the sulfur content in coal obtained by mixing coal with high sulfur content and coal with low sulfur content is measured and the mixing ratio of the coal with high sulfur content and the coal with low sulfur content is automatically controlled.

CONSTITUTION: Coal with high sulfur content and coal with low sulfur content are extracted and mixed from hoppers 2, 4 through rotary feeders 13, 14. A part of the coal after being mixed is sampled through a line 5 and analyzed by an automatic analyzer 6 for the amount of sulfur; on the basis of the output signal 12 from the analyzer the rotating speeds of the feeders 13, 14 are controlled so that the amount of sulfur in the coal supplied to a boiler 8 becomes the target amount of sulfur and the mixed coal is supplied to the boiler 8 and burnt. On the other hand, the concentration of sulfur oxides in the exhaust gas from the boiler 8 is continuously analyzed by an analyzer 11 and its analysis result is cross-checked with the concentration of sulfur oxides in exhaust gas computed by a calculator 10 on the basis of the data from an automatic analyzer 7 for the amount of sulfur. Then, the exhaust gas from the boiler 8 is desulfurized by a desulfurizer and discharged from a stack into the open air.